The Last Great Outboard?

**DBR** · 08-29-2011, 08:34 PM

The Mercury 300xs may be the last great 2 stroke outboard we will see. We hope not. As the world changes and OEM restrictions grow, the 300 XS might be the last production high performance 2 stroke outboard manufactured. The 4 stroke is coming… and as they did in motocross 4 strokes may evolve and eventually perform similar or better than the less expensive more simple 2 stroke. So thanks Mercury, please don’t stop building these engines.

Mercury has done a remarkable job designing and building the 300xs. This Engine is awesome. Box stock this direct inject 3.2 liter engine makes around 305 prop shaft hp and 305 ft lbs of torque. What other engine pushes light boats well over 100 mph for hours on in stock with a warranty? These engines are extremely quite and burn fuel at a rate far better than the conventionally injected 3 liter predecessor. This direct injected design is very simple in concept but quite complicated with pumps, electronics and injectors. We have been testing this engine for over 6 months and have learned a ton with tons to still learn.

The past 10 years we have been modifying, building, testing and successfully racing 3 and 3.3 liter conventionally injected OMC 2 strokes. This education has helped us tremendously with this XS. The XS is also helping us re think our omc program. This XS project is fun, new and very challenging. Hot Roding an old omc is relatively easy as making more power from an antiquated 225 is not that difficult. Squeezing more from a 300xs is not so easy….. so a huge amount of respect and Kudos to the Mercury engineers on building this great engine for us to enjoy. These engines are very quite, easy on fuel and have great tq. with outstanding reliability. Perfect for most boaters but some of us can't leave things alone, we always want more. Below is a little random info we have learned in no special order and hopefully soon we will be sharing more. If we all help each other with what we learn we might save each other a disaster or two.

Reliable:

This engine has taken more testing abuse than we could have ever imagined. Zero failures in hundreds of tests…some tests should have failed but did not. This is evidence of a great stock baseline with latitude either side of the tuning edge.

Cowling:

This is a restrictor plate if your looking for more power. Mercury only intended on 300 hp and if you want to make more you will have to modify or change the cowling with the addition of intake noise. There is always a trade off. We all have seen the cool video Adam Mckeon filmed (thanks) of the high speed water ingestion, obviously not good, but that is only half the story. The lack of air flow do to shear physical opening size coupled with preheating of intake charge creates a noticeable performance loss. We have seen up to 8 hp increase by just removing the cowling on a bone stock engine. We measure the differential in pressure inside and outside of the cowling to monitor flow restrictions. When you modify the engine and make more power this becomes more of a restrictor. As the ambient air temp rises the stock cowling steals more power. When the ambient air is cool it is not as bad. The stock cowling design flows cool air around a warm engine resulting in power loss. Any increase of flow here has helped. Mike Nass has made us 2 styles of intakes vents that work awesome. One is a ram air design and one a direct copy of the 2.5 drag front plate. Soon Mike will have an alien style 300x design that plops onto the 300xs pans. This is needed to make big power. Caution…when you flow more air, the air /fuel ratio changes in the unsafe direction. We have made over 320 hp with stock mapping but temps increase to well above 1400 deg. Mapping changes are necessary if reliability is to be maintained.

Gasoline:
The owners manual says 91 octane and they are right. We have tested 87 crap fuel all the way to high dollar special blends. Very interesting testing fuels in this engine. There is definitely a power loss and slight hint of detonation with 87 octane. I will never put 87 in an xs again. This engine loves 93 and a slight blend of av if you want more power. There is an octane booster that makes 87 good and 91 great. I would have never believed this in a million years but there is a booster that works.

ECU mapping
There is definitely power to be gained in remapping the stock ecu for racing. When running 91 octane or better, trimming some of the multiple curves has resulted in more tq. and hp. Upgrading the maps can actually lower combustion temperature and raise combustion pressure yielding more power while maintaining reliability. If you are looking to increase power you will have to re map. When you make more power you use more fuel. Simple. At 335 hp these engines still burn less than an a 300x. A bunch less.

Gear case:
If you run crazy high engine heights and run lower water pressure you will or may burn up a gear case if you run wide open for minutes at a time. I have friends that run over 115 mph all day and hurt nothing. Some, like us, have hurt a few. Dyno testing is hard on cases... don't dyno unless you have to. Mercury makes the best case. They need to cool down once in a while. Aftermarket seals and venting makes sense.

Tuners:

A lot of people ask us about tuners. What is the Best? So far we have only run the stock tuner and have seen 340 hp with it. We will find out what works and what doesn’t but if you want to keep things quite the stock tuner is very good. We will let you know. WE have built and gathered a bunch to try. This will be interesting and can almost bet if it makes more power it will be louder. We wont know until we try.

Light Fly Wheels:

They work. This is another thing we were surprised to see so much gain. The 300xs compared to a 300x flywheel was a gain and 300xs to a 7 lb aluminum was a big gain. This is the single most noticeable thing in the seat of your pants. The negative to a light flywheel is not as great of an idle with the 7lb as the 18 pound. This change makes the engine snap like a 2.5. It also is not as good at shifting huge props but no where near as bad as a 2.5.

Rev limiter:
Moving the limiter up a few hundred has been safe as we have not seen failures with the limit at 6800. Cast pistons will fail at ????? rpm. The power curve is falling off pretty hard at the stock limiter (6400) but if you like short props and quick blasts moving the limiter Conservative works.

Reeds:

The stock reeds are awesome to 340 hp. Absolutely the best reed. We have found a little more above 6000 but it is a trade off for a loss below 6000 rpm. We have tried a bunch of reeds and again Mercury hit it out of the park here. Stock is best here but we will keep trying.

Sock Air Box:
Run this or an aftermarket stack. If you remove the stock box it kills power.

Hope this might help someone a little. We are working on a bunch of stuff including level 2 and 3.6 liter engines. This will be good…fun. Sorry for the bad spelling.

Dave
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Shop: 928-680-6655
davebushracing@gmail.com
www.davebushracing.com

**JWTjr.** · 08-29-2011, 09:15 PM

Great stuff, Dave. Thanks for posting this. Great to hear detailed information from someone who has a history of demonstrating that he knows what he's talking about because he's done it before and measured the results.

Tiger

**Skater 32** · 08-29-2011, 09:25 PM

Truly thank you for sharing all your hard earned Data with all of us . Do you think mercury dropped the ball on the air intake as it will inhale mist via rear cowl intake ?

Larry

**river rocket** · 08-29-2011, 09:37 PM

I have been on the fence on the 300xs for some time. My current 300x just passed 418hrs, and at some point I will need to make the jump. It’s nice to see such a detailed write up. Thanks for sharing and all your hard work.

**jpf091959** · 08-29-2011, 09:40 PM

Thanks for all the info. Keep it coming. I have a pair of 300xs on my 25 motion. The best I have seen is 110 mph in fresh water.
I have a hanger at the local airport and always run 1/3 to 1/2, 100 octane AV gas with premium pump gas. I did it because the engine seems to run smoother. I have been around planes a for a long time and Aviation fuel is so clean it barely smells at all, how ever it is oxegenated, designed to work at 10,000 ft in elevation, so it is very dry. That is why I mix it with pump fuel. I also run Lucas fuel injector cleaner every once in a while.

The engines and drives have 26 hrs on them now and the last two times I changed the gear oil, there was a little bit of water in counter rotating gear case. Anyone else seeing anything like this. The warranty expires this fall.